1. Field of the Invention
The present invention generally relates to a fuel supply device for direct methanol fuel cells. More particularly, the present invention relates to a fuel supply device for direct methanol fuel cells including a wick structure and a sheet stack. A fuel supply device of the present invention may provide a continuous and uniform supply of liquid fuel in a proper amount and effectively remove by-products.
2. Description of the Related Art
To meet recent demands for portable, small, light-weight electronic equipment, fuel cells are attracting public attention as a battery for electronic equipment. Fuel cells, having a higher energy density than secondary cells, are advantageously used in small, light-weight, electronic equipment. Also, since fuel cells can supply energy to electronic equipment for a long period of time with a single fuel supply, they are suitable for use in portable equipment. Further, fuel cells produce only carbon dioxide and water as by-products. Therefore, fuel cells do not cause air pollution and are environmentally friendly energy sources.
Fuel cells are classified by electrolytes contained in the cells, including, for example, phosphoric acid fuel cells (PAFC), alkaline fuel cells (AFC), polymer electrolyte fuel cells (Proton Exchange Membrane Fuel Cell, PEMFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and direct methanol fuel cells (DMFC).
Fuel cells generate energy by chemical reactions. For example, a chemical reaction of hydrogen ions and oxygen produces heat and water. The heat generated is converted into electric energy and supplied to electronic equipment. However, if hydrogen is used as a fuel, a fuel tank having a relatively large volume is needed. Also, if highly pressurized hydrogen is used to reduce the volume of the fuel tank, a high pressure fuel storing system is needed, which, for safety reasons, is not suitable for use in portable electronic equipment.
Since direct methanol fuel cells use liquid fuel as a hydrogen supply source, direct methanol fuel cells can store much more hydrogen than if hydrogen gas were being stored. Also, direct methanol fuel cells have an advantage in that they may be used for a longer period of time than conventional secondary cells. Further, because direct methanol fuel cells may be readily recharged, they are suitable for use in portable electronic equipment.
As shown in FIG. 1, a direct methanol fuel cell includes a membrane electrode assembly, which includes an anode 2, a membrane 1 and a cathode 3. Reactions occurring in anodes and cathodes of direct methanol fuel cells are shown in the following Reaction Scheme (I).

When methanol is used as a liquid fuel, a methanol cross-over phenomenon occurs, whereby highly concentrated methanol passes through the membrane of a fuel cell without ionization. Such a cross-over phenomenon may cause efficiency of the fuel cell to deteriorate, since the supplied fuel is consumed without participating in the reaction. Therefore, in order to reduce the cross-over phenomenon, methanol is diluted with water in a proper ratio to control the concentration of methanol in the liquid fuel at a predetermine level, before being supplied. However, since water mixed with methanol is surplus moisture and is not needed for the chemical reaction to generate energy, it should be smoothly discharged off to improve the efficiency of the fuel cell. Also, the carbon dioxide generated as a by-product of the chemical reaction should be effectively removed from the membrane. Otherwise, the chemical reaction in the fuel cell is hindered by the carbon dioxide and the fuel cell efficiency reduced.
Therefore, there is a demand for a fuel supply device that continuously supplies a proper amount of liquid fuel to direct methanol fuel cells and effectively removes carbon dioxide and water generated in the fuel cells.
A related solution provides a fuel supply device including a wick structure to supply liquid fuel to an anode of a fuel cell through the capillary phenomenon between the fuel cell and a fuel storing tank. However, this fuel cell device solve the problem associated with the methanol cross-over phenomenon, since the anode is in contact with the liquid fuel through the wick structure, whereby an excessive amount of the liquid fuel is supplied in a short time. Also, this fuel cell device cannot effectively remove carbon dioxide and water generated as by-products in the fuel cell, causing deterioration of the efficiency of the fuel cell.
In addition, some passive fuel supply devices using a porous medium are known. Fuel cells employing these passive fuel supply devices using a porous medium are largely affected by the performance of the porous medium in terms of fuel cell efficiency. The porous medium transports and supplies a liquid fuel from a fuel tank to the fuel cell by capillary force through direct contact with an anode of the fuel cell. Therefore, quality of the porous medium is determined by porosity and pore size distribution. If the pore size distribution of the porous medium is large, it is difficult to uniformly supply liquid fuel to an electrode surface and to remove by-products such as carbon dioxide and vapor. When transportation of liquid fuel to the membrane through the electrode is hindered due to carbon dioxide and vapor, the efficiency of the fuel cell is hastily deteriorated and fuel leakage may occur.